Magnetic flowmeters, in which a conductive fluid flows through a transverse magnetic field of given density to produce a voltage between a pair of electrodes that is proportional to the flow rate, have been used for some time. These meters operate on the well established principle that the voltage generated is perpendicular both to the magnetic field and to the direction of flow.
A majority of the commercially available magnetic flowmeters contain two small electrodes that are flush mounted within a flow conduit on opposite sides thereof. A magnetic coil and core assembly are externally mounted on the conduit so as to create a magnetic field that passes through the conduit substantially at right angles both to the flow and to the position of the electrodes.
It has also been proposed to create an internal, concentric magnetic field by centering a current-carrying conductor along the longitudinal axis of the conduit (i.e., aligned with the direction of flow). In this type of axial-current magnetic flowmeter, referred to as an integral (i.e., self-contained) field magnetic flowmeter, the output voltage is developed along an axis perpendicular both to the direction of flow and the magnetic field and detected between a pair of concentric electrodes located within the field. For magnetic fields generated by such an axial current, the field intensity at any point in the flow conduit is inversely proportional to the radial distance of that point from the effective mean axis of the current.
One form of flowmeter utilizing an integral field generated by a current-carrying conductor is shown in an article published in the June 1970 issue of Instrumentation Technology entitled "A Magnetic Flowmeter with Concentric Electrodes" by P. C. Eastman et al. In that device, a metal tube enclosing the conductor is welded into a pipe for use as a central electrode. A cylindrical outer electrode mounted to the pipe wall surrounds the conductor. Since the tube is electrically grounded to the pipe, a non-conductive liner must be attached to the interior of the pipe to insulate the outer electrode therefrom to establish a potential across the two electrodes for sensing the flow induced voltage. Liners are expensive to manufacture and install, and pose significant problems with respect to bonding and sealing under certain flow rate conditions, and therefore are not readily adaptable for placement in existing pipe lines in a process control field.
In the design of a magnetic flowmeter, the magnetic field generated may be either alternating or direct. However, the use of an a-c field excitation increases the sensitivity of the measured output flow rate signal to variations in line voltage, frequency and harmonic distortions, as well as inductively or capacitively coupled spurious voltage signals unrelated to flow rate. In addition, large a-c input current levels are required to produce a field of sufficient intensity, involving an attendant increase in energy dissipation.
If direct magnetic flux is employed, other drawbacks are encountered. The d-c signal current tends to cause polarization of the electrodes that builds up proportionally with the duration of the current. This polarizing effect increases the ohmic impedance of the electrodes and has a more significant effect on electrodes having small surface areas.
Also electrochemical or galvanic potentials that vary with time are present at the electrodes. In fact, under certain flow conditions significant changes in these voltages can occur in fractions of seconds. Since the flow rate signal is superimposed on these electrochemical voltages, some means must be provided for separating the two voltages, or compensation for the shift in the baseline must be built into the measurement circuitry to preserve output accuracy.
An additional method for creating the magnetic field is disclosed in U.S. Pat. No. 4,010,644 in which a pulsed d-c signal current is utilized. Such a method has certain advantages over a-c or constant d-c field excitation, namely reducing energy consumption and minimizing the effects of a-c line voltages, inductively coupled spurious output signals, and polarization. However, because of the long time constants associated with the field windings of that prior art device, the effects of shifting electrochemical voltage must be taken into account. Thus, the aforementioned U.S. Pat. No. 4,010,644 is primarily concerned with a method of compensating for these perturbing d-c voltages when using a pulsed d-c field. However, this method involves additional expenditure in circuitry and overall complexity for making the true flow rate measurement.
Still another factor associated with prior art devices is the relatively small size of the sensing electrodes. As mentioned, small electrodes are more susceptible to the effects of polarization. Additionally the produce high output signal impedances that interfere with the ability to measure the flow rate of low conductivity fluids.
Still further, small electrodes generate noisy flow signals that affect output accuracy. This is due to the fact that an element of flow induced voltage in the immediate vicinity of an electrode contributes a disproportionately larger percentage of the total flow rate signal than the same voltage induced in the flowing liquid at a point more remote from the electrode. Since practical flows are almost always turbulent, random velocity variations occur near the electrode which do not reflect the true mean flow velocity in the conduit. Thus magnetic flowmeters using relatively small electrodes produce output flow signals with typically several percent of random noise superimposed which must be filtered to produce usable results. This filtering slows the response of the measurement system.